Field of the Invention
The present invention relates to a sensor system, a substrate handling system and a lithographic apparatus.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic process accurate positioning of the substrate is required in order to properly transfer the pattern from the patterning device to the substrate.
In a known method to align the substrate, the alignment is done in two steps. In a first step, a pre-alignment step is done to correct for coarse angular and translational mis-positioning. In a second step, die alignment is done that provides precision location keyed to specific features on the individual semiconductor die.
A known method of pre-alignment is to transfer the substrate onto a pre-alignment substrate table where it is rotated while the radial distance from substrate edge to center of the substrate table is measured. The sequence of these radial measurements is used to determine the centering of the substrate on the substrate table, a translational position of the substrate, and the location of marks or notches on a periphery of the substrate which defines a rotational orientation of the substrate. The movement of the substrate table assures rotational alignment, and the gripping device that then transfers the substrate to the substrate table where the actual lithographic process takes place compensates for its translational misalignment.
In this way, pre-alignment system may for example be able to align the substrate to within a degree of rotation and ten thousandths of a centimeter in translation.
In a known embodiment of a sensor system configured to determine the position of a substrate edge, the sensor system comprises a radiation source and a detector device, wherein the sensor and the detector device are arranged at opposite sides of the substrate.
In such sensor system, typically the detector device is arranged below the substrate and the radiation source, also called back radiation source is arranged above the substrate. As a result, energy transmitting cables and/or radiation transmitting cables have to be provided above the substrate which may be disadvantageous as the available space above the substrate is limited. Also, the presence of the radiation source above the substrate may result in heat dissipation and ineffective radiation radiation at an undesirable location. Further, the presence of cables and a radiation source above the substrate may create an obstruction of air downflow on the substrate.
Another known sensor system is disclosed in Unite States patent application US2007/0045566A1. This sensor system is arranged to determine the position of the edge of a substrate. The sensor system has a light source and a sensor. The sensor receives light reflected from the bottom surface of the substrate. However, the shape of the bottom surface may be different near the edge than elsewhere on the substrate. The difference in shape changes the optical properties of the substrate near the edge. Because of the change in optical properties, the known sensor system does not provide a defined way to align a wafer.